1. Field of the Invention
The present invention relates to a projection apparatus, a light source apparatus, and a projection method preferable for a projector using a high-resolution optical modulation element.
2. Description of the Related Art
In television receivers and displays, products compatible with 4K resolution (a resolution in the order of 4000 pixels wide×2000 pixels high) are becoming widely available. Jpn. Pat. Appln. KOKAI Publication No. 2012-242626 proposes a similar technique of making a projection apparatus such as a data projector compatible with high resolutions.
Techniques have been considered, including the technique described in the above-described patent literature, to achieve a higher resolution in a projection apparatus that irradiates an optical modulation element with light from a light source, forms an optical image using its transmitted light or reflected light, emits the optical image via a lens optical system, and projects the optical image onto a screen surface on which projection is performed.
Of such techniques, consideration will be given to a projector apparatus based on Digital Light Processing (DLP) (trademark), which uses, as an optical modulation element, a micromirror element that has been put into practical use by microelectromechanical systems (MEMS), a technology that is becoming more available, particularly in recent years.
When a projector apparatus compatible with, for example, 2K resolution (a resolution in the order of 2000 pixels wide×1000 pixels high) is already commercially available and a projector apparatus that uses a micromirror element compatible with 4K resolution is newly commercialized, a higher resolution can be achieved very strategically if the element size of the micromirror element does not need to be changed, since hardware items other than the micromirror element, such as a light source system and a lens optical system, in the periphery of the micromirror element do not need to be redesigned in accordance with an increase in element size.
To achieve higher resolution without changing the element size, however, the area of micromirrors per one pixel is, as a matter of course, reduced to approximately ¼, for example. In a micromirror element comprising such micromirrors with an extremely small area arranged in an array, it is known that, particularly under a drive environment in which the same on or off state is maintained in an area of an image for a long period of time, a problem of adhesion of micromirrors occurs in that area.
In a projector apparatus equipped with a feature of keystone correction to an image, for example, an off display state needs to be maintained to reflect light from a light source of somewhere other than a lens optical system, so as not to display an image in triangle-shaped areas at both edges of the image. In such a case, there is a high possibility that adhesion of micromirrors occurs in those areas.
To prevent such adhesion of the micromirrors, it has been confirmed that a refresh operation of switching on and off micromirrors to reverse the displayed content for the duration of over 100 μs, which roughly corresponds to, for example, 1/100 of the entire length of every one or two frames of image projection, is effective.
When a projection operation is continued during the refresh operation, however, an operation of projecting an image other than the original projection image is continuously performed in areas in which both edges are cut by the keystone correction feature, for example. This causes a significant decrease in quality of the projection image. It is thus necessary to provide, at the time of the refresh operation, a black interval, during which all the light sources are temporarily turned off.
Hereinafter, a description will be made of the process of setting a black interval to allow a micromirror element to perform a refresh operation. In the description that follows, assume that a single-chip projection apparatus, which uses one micromirror element and a fluorescent wheel, is provided.
The video frequencies recognized by projector apparatuses are mainly based on the National Television System Committee (NTSC) system, which is a system (standard) for composite video signals developed by the Committee and their television broadcasts, and the Phase Alternating Line (PAL) system, which is a standard for color composite video signals. The vertical synchronization frequency is 60 Hz (or more correctly, 59.94 Hz) in the NTSC system, but is 50 Hz in the PAL system. It is thus necessary to cope with operations of a plurality of input signal frequencies.
FIG. 6A shows a fluorescent wheel rate in the case where an input signal of 52.1 Hz, which is the highest frequency in the setting range of the 50 Hz system, is frequency-locked. Since the apparatus is driven at a frequency double the frequency of the input signal, the frequency inside the apparatus is 104.2 Hz, double the frequency of the input signal. In FIG. 6A, the bottom row shows the primary color fields of red (R), green (G), and blue (B), and the top row shows synchronization pulses that are synchronized with the fields. The synchronization pulses are generated such that a fluorescent wheel is synchronized with the fields, as shown at the bottom row of FIG. 6A, and corresponding light-emitting elements are synchronized and driven to emit light. Thus, primary color light is properly emitted in a time division manner and a micromirror element is irradiated with the primary color light. In the micromirror element, image display is performed according to the irradiated primary color light, thus forming an optical image by its reflected light and projecting the optical image via a lens optical system.
On the other hand, FIG. 6B shows a fluorescent wheel rate in the case where an input signal of 62.0 Hz, the highest frequency in the setting range of the 60 Hz system, is frequency-locked. Since the apparatus is driven at a frequency double the frequency of the input signal, the frequency inside the apparatus is 124.0 Hz, double the frequency of the input signal. Similarly, in FIG. 6B, the bottom row shows the primary color fields of red (R), green (G), and blue (B), and the top row shows synchronization pulses that are synchronized with the fields.
In each of the fluorescent wheel rates, a permissible range within which the frequency can be locked is set by the apparatus, as described above. In this case, assume that 94.00 Hz-104.20 Hz are set as a permissible range of the 50 Hz system, and 102.40 Hz-124.00 Hz are set as a permissible range of the 60 Hz system. As described above, since circuits of the apparatus are driven at a frequency double the frequency of the input signal, the frequency inside the apparatus is double the frequency of the input signal. If the input signal has a frequency out of the above-described set range, the apparatus cannot lock the input signal, and thus cannot perform a projection operation.
FIGS. 6A and 6B show timing in the case where the frequency is locked to the highest frequency in the permissible range.
FIG. 7 shows a fluorescent wheel rate in the case where an input signal of 50 Hz, which is a standard value of the 50 Hz system, is locked. The synchronization frequency inside the apparatus is 100.0 Hz, and a period of one image frame is exactly 10000 μs.
In these drawings, the ratio of the field intervals of R, G and B is set to 1:1:1, for ease of explanation. However, the frequency-locked signal can be handled even when other ratios are used, without losing the balance.
As described above, the duration of one frame period differs according to the frequency of the input signal to be locked. For example, in the 60 Hz system, one period is 8064.5 μs when the frequency is 124.0 Hz, and one period is 9765.6 μs when the frequency is 102.4 Hz. Thus, the period is approximately 21% longer at the lowest frequency, than at the highest frequency.
Similarly, in the 50 Hz system, one period is 9596.9 μs when the frequency is 104.2 Hz, and one period is 10638.3 μs when the frequency is 94.0 Hz. Thus, the period is approximately 10% longer at the lowest frequency, than at the highest frequency.
The processing time for performing the refresh operation varies according to the situation in which an input signal is locked. Here, the duration of the refresh interval is defined as follows:Time length=(Maximum frequency of fluorescent wheel rate/Minimum frequency of fluorescent wheel rate)×100 μs
Then, the duration of the refresh interval in the 60 Hz system is approximately 121 μs (≈(124.0/102.4)×100 μs), and the duration of the refresh interval in the 50 Hz system is approximately 111 μs.
To describe execution of the refresh operation, the conventional sequence operation will now be described.
FIG. 8A is a timing chart showing a usual fluorescent wheel rate, in which a refresh operation is not executed. The top row shows synchronization pulses applied to the power supply, and the bottom row shows primary color light emitted by the light source driven by the power supply.
Assume that red light R is obtained by an independent light source (second light-emitting element) of an LED that emits red light, and green light G is obtained as fluorescent reflected light by irradiating a fluorescent material applied to the fluorescent wheel shown in FIG. 8B (range “G” in the drawing) with blue laser light, and blue light B is obtained as transmitted light by allowing the blue laser light emitted from an independent light source (first light-emitting element) to pass through a diffuser panel of the fluorescent wheel shown in FIG. 83 (range “B” in the drawing).
When synchronization pulses shown at the top row of FIG. 8A are issued toward the power supply, the power supply that has received the pulses performs the following processing.
That is, the power supply that has received a synchronization pulse (1) turns off the red LED, and turns on the blue laser at the same time. In this case, the range G of the fluorescent wheel is irradiated with blue laser light, and green light is obtained as its fluorescent reflected light.
The power supply that has received a synchronization pulse (2) adjusts the current value by adjusting the color balance, as needed, while keeping the blue laser turned on. In this case, the light source separately and simultaneously performs synchronization adjustment of the fluorescent wheel in such a manner that, as the fluorescent wheel rotates in the direction D, the diffuser plate B is irradiated with the blue laser light at timing t11 shown in FIG. 8A, and blue light is obtained as its transmitted diffusion light.
The power supply that has received a synchronization pulse (3) turns off the blue laser, and turns on the red LED at the same time. Red light is obtained as an independent light source.
A case will be considered where a black interval is inserted to the sequence shown in FIG. 8, based on the above-described refresh operation.
FIG. 9 shows an example of a virtual fluorescent wheel rate in which a black interval is arranged at the end of the frame period in the sequence shown in FIG. 8A. If such an operation can be implemented, the refresh operation can be easily executed.
FIG. 9 is the same as FIG. 8A, with regard to the synchronization pulses (1) and (2) and the interval during which the red light R is obtained, and the interval during which the green light G is obtained.
In addition, a black interval is formed between a synchronization pulse (3)′ and a synchronization pulse (4)′ by deleting an angle of 6° from a central angle of 120° of the fluorescent wheel, during the interval of blue light B shown in FIG. 8A.
In practice, when a black interval is formed while maintaining the color balance, the black interval needs to be moved to the synchronization pulses (1) and (2), in accordance with the proportion each color constitutes. To form a black interval corresponding to an angle of 6° when the time ratio of R, G, and B is 1:1:1, namely, the ratio of the central angle of the fluorescent wheel of R, G, and B is 120°:120°:120°, for example, the synchronization pulses (1)-(3) are generated such that the ratio of R, G, and B becomes 118°:118°:118°.
To form a black interval corresponding to an angle of 6° when the time ratio of R, G, and B is 3:2:1, namely, the ratio of the central angle of the fluorescent wheel of R, G, and B is 180°:120°:60°, for example, the synchronization pulses (1)-(3) are generated such that the ratio of R, G, and B is 177°:118°:59° to maintain the color balance. The operation performed by the power supply that has received the synchronization pulses (1) and (2) is similar to the example of FIG. 8A. On the other hand, the power supply that has received the synchronization pulses (3)′ and (4)′ needs to operate as will be described below.
That is, the power supply that has received the synchronization pulse (3)′ turns off the blue laser.
After that, upon receipt of the synchronization pulse (4)′, the power supply turns on the red LED.
FIG. 10A is an enlarged view of the synchronization pulses (3)′ and (4)′. The amount of light emission from a blue laser is not immediately reduced to zero by its responsiveness when the blue laser is turned off by the synchronization pulse (3)′, and the amount of light emission from a red LED does not immediately reach 100% by its responsiveness when the red LED is turned on by the synchronization pulse (4)°.
FIG. 10B shows an example in which a time interval of 160 μs is secured between the synchronization pulses (3)′ and (4)′. It requires approximately 40 μs until the amount of light emission from the blue laser is actually reduced to zero after the blue laser is turned off by the synchronization pulse (3)′, as described above, thus securing a black interval of approximately 100 μs.
When using a product that has poor blue level responsiveness and that requires further time until the amount of light emission is reduced to zero after the blue laser is turned off, the interval between the synchronization pulses (3)′ and (4)′ can be further increased.
In this case, the problem is that the temporal interval between the synchronization pulses (3)′ and (4)′ is extremely short. When the synchronization pulses (3)′ and (4)′ are generated and are applied to a digital power supply that is used for this type of product, the power supply side cannot recognize the latter synchronization pulse (4)′, due to the extremely small temporal interval.
Typically, in a digital power supply used for this type of projection apparatus, synchronization pulses need to be separated by 500 μs at minimum. To achieve the operation described above with reference to FIG. 9, the digital power supply used in a conventional product needs to be replaced with a more expensive power supply, or a power supply needs to be newly developed.
Thus, the feasibility of the operation of generating a black interval for a refresh operation using synchronization pulses, as described above with reference to FIG. 9, is very low. Approaches have been attempted to generate an appropriate black interval according to the frequency of an input signal using other means.
The present invention has been made in view of the above-described circumstances, and an object of the present invention is to a projection apparatus, a light source apparatus, and a projection method capable of inserting, during a projection time, a black interval, a very short period of time during which projection is not performed, according to the frequency of an input video signal, without a significant change to circuits, etc., constituting the apparatus.